1. Technical Field
The present invention relates to a light source device, a control method therefor, a lighting device, a monitor device, and an image display device.
2. Related Art
In the past, a UHP lamp has been used as a lighting beam source for a projector, but has problems regarding restriction in the color reproducibility range, instant lighting, a life, and so on. Therefore, use of a semiconductor laser as the lighting beam source for a projector has been proposed. However, since sufficient intensity of light in the visible wavelength range cannot be obtained directly from a semiconductor laser source, a second harmonic generator (SHG) for converting an infrared wavelength into a half wavelength thereof is used concomitantly with the semiconductor laser source. Further, in order for obtaining a stable light output, feedback control of a drive current of a semiconductor laser has also been performed (see e.g., JP-A-9-232665).
However, in the past configuration, there is no designation regarding timing of laser current control or temperature control of the light source device, and in the case in which a calibration of the light intensity is performed while operating as a light source, there is caused a problem that in the calibration process, the intensity of emitted light temporarily decreases or increases for a time period recognizable to the observer depending on whether or not the drive condition is appropriate, thus an intensity variation in the lighting beam is observed.
Further, the light source device is mainly composed of a resonator to which a specific wavelength is set as a resonant wavelength and a wavelength conversion element for converting the wavelength set to the resonator. However, it is known that the oscillation wavelength of the light source device shifts in accordance with the temperature, and it is a challenge to obtain a high power output with a stable oscillation wavelength.
On the other hand, it is known that the wavelength conversion element, which has a function of converting the wavelength of the input infrared laser into, for example, a half wavelength, causes energy absorption of about 1E−2 through 1E−4 of the infrared light, resulting in generation of temperature variation dependent on the input infrared light beam.
P=∈0χ(1)E+∈0χ(2)EE+∈0χ(3)EEE+ . . . =P(1)+P(2)+P(3)+ . . . (P: second-harmonic power, ∈0: electric constant in vacuum, χ(1), χ(2), χ(3) . . . : nonlinear susceptibility, E: energy density per limit area).
The formula described above represents the energy relationship in executing the wavelength conversion on the incident light beam, and since the high order energy such as second-order or third-order is added, it can be confirmed that high-power energy injection contributes to achievement of high efficiency. When viewed from the opposite side, if low energy is injected, loss of energy as heat absorption becomes large, thus the heating energy of the wavelength conversion element should be increased.
On the other hand, the temperature range in which a PPLN, the wavelength conversion element, operates at high efficiency is typically within one degree, which requires high-accuracy temperature control, and the shrinkage and expansion of the wavelength conversion element caused by the temperature variation according to the variation in the condition of the infrared laser beam makes the efficiency variation thereof large.
As described above, in realizing the light source capable of operating at high efficiency, it is necessary to optimize the drive condition of the laser emission source so that the wavelength shift and reduction of emission efficiency dependent on the temperature in the laser source can be suppressed, and the wavelength conversion efficiency in the wavelength conversion element can be maintained. As shown in FIG. 11, the relationship between elapsed time and a drive condition (e.g., a pulse width) in a past pulse drive condition setting changes continuously from the present condition, and is switched to an optimum power B by automatic power control from a time point A when the optimum point is detected. Since the relationship changes continuously, the light intensity variation is confirmed by the observer to make the observer feel the light intensity variation of a picture, thus a preferable picture can hardly be obtained.